The present invention relates to a cooling system for a metallurgical furnace, in particular a blast furnace.
Known blast furnace cooling systems are cooling water circuits, in which cooling water is circulated in a closed circuit by electric circulation pumps. The elements of the blast furnace to be cooled (i.e. the cooling staves and cooling boxes of the furnace walls, the tuyeres and hot blast equipment) are regrouped in several parallel branches or sub-circuits, which are hydraulically balanced so that a predetermined flow of cooling water circulates through each sub-circuit. A common return line, comprising one or more heat exchangers, closes the cooling circuit.
In case of an electric power failure the cooling is interrupted because the electric circulation pumps do not work. To protect cooled elements against damages in such a case, it is known to provide an emergency cooling system. Such an emergency cooling circuit comprises a gravity tank that is mounted on a support structure that is higher than the blast furnace. An emergency feed line, which is designed for a very low pressure drop, connects this gravity tank to the cooling water circuit of the blast furnace and is provided with an emergency feed valve. An emergency cooling water overflow with an emergency overflow valve is provided at the highest point of the closed cooling circuit. In case of an electric power failure, the emergency feed valve and the emergency overflow valve open. Gravity pushes the water reserve contained in the gravity tank into the cooling circuit of the blast furnace. At the highest point of this cooling circuit, the cooling water is discharged of the cooling circuit through the open emergency overflow valve into a receiving tank. In summary, emergency cooling takes place by gravity in an open circuit until the gravity tank is empty. A high pressure pump station is required to refill the gravity tank. As this high pressure pump station is generally equipped with electrical pumps, the refilling operation can only start after the end of the power failure. It will be noted that the cooling system is without effective emergency cooling function until the gravity tank is refilled.
In order to reduce the storage capacity of the emergency gravity tank, it is known to provide an emergency pump with an internal combustion engine in the closed cooling circuit. In this case it is theoretically sufficient to dimension the storage capacity of the gravity tank to bridge the time needed for starting the emergency pump. Once the emergency pump has started, the emergency feed valve and the emergency discharge valve are closed so that the cooling system works again as a closed circuit.
It will be noted that such an emergency cooling system is quite expensive. Important cost factors are not only the gravity tank and its support structure, but also the big diameter emergency water pipe, which may be several hundred meters long. In this context it will be noted that the emergency pump may help to reduce the costs of the gravity tank itself, but has of course no influence on the costs of the big diameter emergency water pipe.
It is also well known that frost protection for the gravity tank and the feed line up to the emergency feed valve often causes serious problems. Furthermore, as the emergency water is often charged with solid corrosion particles and algae, the cooling circuits of the blast furnace are contaminated after an emergency water discharge. It follows that the cooling circuits must be rinsed thoroughly after each emergency water discharge. This is in particular troublesome, if short electric power failures triggering a discharge of the emergency cooling system are quite frequent.
It is an object of the present invention to provide a cooling system for a that is less expensive but nevertheless more reliable than existing cooling systems on metallurgical furnaces.
A metallurgical furnace cooling system in accordance with the present invention includes a cooling circuit comprising an inlet and an outlet for cooling water. A return line connects the outlet to the inlet so as to form a closed cooling circuit with at least one circulation pump for circulating cooling water through this closed circuit. An emergency feed line with an emergency feed valve is connected to the inlet of the cooling circuit. This emergency feed valve opens in case of a power failure. At its highest point, the closed cooling circuit is equipped with an emergency overflow valve, which opens in case of a power failure, so that the closed cooling circuit becomes an open cooling circuit with an atmospheric pressure discharge at its highest point. In accordance with an important aspect of the present invention, the emergency water gravity tank is replaced by a pressure vessel means connected to the emergency feed line. This pressure vessel means contains a certain volume of emergency water that is pressurised by a pressurised gas. The gas pressure in the pressure vessel means warrants that an emergency water flow establishes through the open cooling circuit, in the direction of the emergency overflow valve, when the emergency feed valve and the emergency overflow valve open in case of a power failure. It will be appreciated that such a cooling circuit is a solution to a long-felt need for a cooling system for metallurgical furnaces, in particular blast furnaces, with an emergency cooling function, which is less expensive than the gravity tank solution, but nevertheless more reliable. As the pressure vessel means need not be mounted on a support tower that is higher than the blast furnace, it can be located much closer to the blast furnace, so that the emergency feed line gets shorter. Furthermore, the diameter of the emergency feed line can be reduced, because: (1) this line is shorter; and (2) a higher pressure drop in this line can be easily compensated by a higher gas pressure in the pressure vessel means. It follows that important savings can be made with regard to the costs of the emergency feed line. Further cost savings are due to the fact that an high pressure pump station, which is needed for refilling a gravity tank, becomes superfluous. Indeed, the pressure vessel means of a cooling system in accordance with the present invention can be easily refilled when the tank is depressurised, so that no high pressure pump station is necessary. After refilling with water, the pressure vessel means can be repressurised by injection of a pressurised gas. It will be appreciated that in blast furnace or steel making plants, pressurised nitrogen is normally available in the required quantities and at the required pressure for rapidly pressurising the pressure vessel means. With the system in accordance of the invention it is consequently possible to have two or more successive emergency water discharges to bridge the time laps until the end of the power failure or until the start of an emergency pump or an emergency power unit. Accordingly, the water reserve in the pressure vessel means can be much smaller than in a gravity tank. It will further be appreciated that freezing protection is easier with pressure vessel means that are located close to ground level and close to the cooling circuit, than with a high gravity tank located further away from the blast furnace. Another advantage is found in the fact that the pressurised gas in the pressure vessel means, which is generally nitrogen, avoids that the emergency water comes into contact with the atmosphere, which is of course of advantage with respect to water quality and corrosion problems. It follows that it can be expected that the emergency water from the pressure vessel means will be normally free of solid corrosion particles and algae and that contaminate of cooling circuits after an emergency water discharge will be the exception.
In accordance with another important aspect of the present invention, the pressure vessel means is not only used as pressurised emergency water reserve, but also as pressurised make-up water reserve, which advantageously replaces a make-up water reserve and a make-up water pump. In this case, the system further comprises a make-up water injection line with a make-up water injection valve connected between the closed cooling circuit and the pressure vessel so as to be capable of injecting pressurised emergency water from the pressure vessel as make-up water into the closed cooling circuit. This solution does not only provide important cost advantages, it also warrants that the emergency water reserve is regularly renewed, which has of course a positive repercussion on the quality of the water in the tank.
The pressure vessel means will be normally equipped with: a gas line and a gas supply valve, for supplying a pressurised gas into the pressure vessel means; a make-up water line and a make-up water valve, for supplying make-up water to the pressure vessel means; and a vent line with a vent valve for relieving gas pressure from the pressure vessel means.
In order to save make-up water and to reduce the refilling time of the pressure vessel means, the cooling system as advantageously includes reservoir means located higher than the pressure vessel for collecting the cooling water flowing through the open emergency overflow valve and an emergency water return line with an emergency water return valve connecting the reservoir means to the pressure vessel means.
In order to reduce the gas pressure in the pressure vessel means, the latter may comprise a pressure vessel that is located at a certain height above ground, for example at the top of a cowper.
In order to reduce the time between two successive discharges and to make thereby the emergency cooling even more reliable, the pressure vessel means advantageously comprises a first and a second pressure vessel that are connected in parallel to the emergency feed line. This cooling system then further includes: a first gas line connected through a first gas valve to the first pressure vessel, for supplying a pressurised gas into the first pressure vessel; a second gas line connected through a second gas valve to the second pressure vessel, for supplying a pressurised gas into the second pressure vessel; a first vent line with a first vent valve for venting the first pressure vessel; a second vent line with a second vent valve for venting the second pressure vessel; an emergency water return line collecting the cooling water flowing through the open emergency overflow valve; a first emergency water return valve connecting the emergency water return line to the first pressure vessel; a second emergency water return valve connecting the emergency water return line to the second pressure vessel; and a pressure equalising line with a pressure equalising valve connected between the first and the second pressure vessel. This system allows to recuperate at least part of the pressurising gas after for a subsequent emergency discharge and to reduce thereby the time required for re-pressurising the pressure vessel means after a discharge. It enables to bridge the time laps until the end of the power failure or until the start of an emergency pump or an emergency power unit by successive emergency water discharges of the first and the second pressure vessel. It follows that the two pressure vessels can be designed for containing a rather small volume of emergency water, without affecting the reliability and effectiveness of the emergency cooling function.
It will also be appreciated that the present invention provides a blast furnace cooling circuit design which makes it possible to considerably reduce the piping costs. Such a blast furnace cooling circuit comprises at least a first sub-circuit connected to at least a second sub-circuit by means of at least one booster pump.
Another important aspect is a closed expansion vessel connected to the closed cooling circuit, wherein the closed expansion vessel is pressurised with a gas. This solution enables a better pressure control and has a positive aspect on water quality.
A cooling system in accordance with the invention normally includes several electrical circulation pumps and at least one emergency pump powered by a thermal engine mounted in parallel with the electrical circulation pumps. Alternatively, it may also comprise an emergency power generation unit for powering at least one of the electrical circulation pumps.